Crucial dependence of excimer laser toughness of “wet” silica on excess oxygen

Creation of point defects by ArF (6.4 eV) and F₂ laser (7.9 eV) irradiation in synthetic “wet” silica glass thermally loaded with interstitial O₂ molecules was studied by optical absorption, electron paramagnetic resonance and infrared absorption. The presence of excess oxygen caused a significant increase of laser-induced ultraviolet (UV) absorption, which was 4 times (7.9 eV-irradiation) and > 20 times stronger (ArF irradiation) as compared to O₂-free samples. The spectral shape of photoinduced absorption nearly completely coincided with the spectral shape of oxygen dangling bonds (NBOHC) in 3 to 6.5 eV regions. The contribution of Si dangling bonds (E' centers) was less than few % and was not dependent on oxygen content. Peroxy radical defects were not detected. The photoinduced NBOHCs thermally decayed at 400...500 C. However, a subsequent brief 7.9 eV irradiation restored their concentration up to 70%. This sensitization can be in part attributed to generation of interstitial Cl₂ and HCl. These data show that oxygen stoichiometry is an important factor for maximizing laser toughness of wet silica.     

Origins of optical absorption between 4.8 and 4.9 eV in silica implanted with Si and with O ions

We have determined some of the sources of the optical absorption bands between 4.8 and 4.9 eV in Si and O-implanted silica using several ion energies to produce layers of implanted ions with constant concentrations. The concentrations of implanted ions in the implanted layers ranged from >0.015 at.% to <3 at.%. Optical absorption was measured from 2.0 to 6.5 eV. Electron paramagnetic resonance measurements were made at ∼20.3 and 33 GHz for sample temperatures ranging from 77 to 100 K for most measurements. The components identified in the spectra, based on comparison with reported parameters, were, in the Si case, due to E′ centers and peroxy radicals. In the O case they were due to E′ centers, non-bridging oxygen hole centers, peroxy radical centers, and a newly appearing state which we labeled the OS center. By comparing the changes in the absorption at 4.83 eV with the changes in the concentrations of the various electron paramagnetic resonance components and with the reports in the literature, we conclude that there are at least four oxygen related centers and one Si related center absorbing between 4.8 and 4.9 eV.     

Optical investigations of germanium nanoclusters – Rich SiO2 layers produced by ion beam synthesis

In this work, refractive index and extinction coefficient spectra of germanium nanoclusters – rich SiO₂ layers have been determined using variable angle spectroscopic ellipsometry (VASE) in the 250–1000 nm range. The samples were produced by Ge⁺ ion implantation into SiO₂ layers on Si substrates and subsequent annealing at temperatures from 700 to 1100 °C. It is known from previous investigations of similar samples that the Ge nanoclusterization process starts already at 800 °C and spherical Ge nanocrystallites 5–8 nm in diameter are observed in the SiO₂ layers after annealing for 1 h at even higher temperatures of 1000–1100 °C. Rutherford backscattering spectrometry (RBS) was employed to measure the Ge atom concentration depth profiles in the studied samples. The RBS results helped us choose realistic models for the VASE analysis which were necessary for a proper interpretation of the VASE data. It has been found that the refraction index value for the SiO₂/Si layer increases after Ge implantation. This effect can be explained by a defect-dependent compaction of ion-bombarded layers. A band’s tail in the extinction coefficient spectra for all the samples is observed which originates from a strong ultraviolet absorption band at 6.8 eV due to a Germanium Oxygen-Deficiency Center (GeODC) and/or a Ge-E’center in SiO₂. The annealing process results in the emergence of weaker extinction coefficient bands in the 400–600 nm region, associated with direct band-to-band transitions in Ge nanostructures. Transformation of these bands, including their blue-shift with the increasing annealing temperature could be explained via a quantum-confinement mechanism, by size and structural changes in Ge nanostructures.     

Transient optical absorption in pulsed-X-ray-irradiated pure-silica-core optical fibers: Influence of self-trapped holes

We investigated pulsed-X-ray-induced attenuation in two pure-silica-core (PSC) optical fibers with low-chloride (Cl) and different hydroxyl (OH) contents. We measured at room temperature the temporal (100 ms–1000 s) and spectral (0.73–3.1 eV) variation of the induced optical absorption after a 1 MeV X-ray pulse in both low- and high-OH PSC multi-mode fiber samples. A component of the transient loss in the low-OH sample was found to comprise absorption bands at 1.63 and 1.88 eV (760 and 660 nm) arising from self-trapped holes (STHs). Already known to be unstable at room temperature, STHs appear to play a key role in the transient responses of low-OH/low-Cl pure-silica-core fibers in the visible/near-infrared part of the spectrum.     

Optical studies of aromatic polyazomethine thin films

Optical transmission and fundamental reflectivity spectra of aromatic polyazomethine thin films, obtained by vacuum evaporation via the polycondensation process have been investigated in the wide spectral range 0.49–6.2 eV. As initial monomers, terephaldehyde (TPA) and one of four different amines, i.e. paraphenylene-diamine (PPDA), 7,2-diamino-fluorene, 1,1′-biphenyl-3,3′,4,4′-tetramine and 3-amino-4-(1-naphthyldiazenyl)phenylamine (fat brown RR) have been used, respectively. Amorphous character of these films was confirmed by the results of X-ray diffraction and AFM investigations. Absorption coefficient of the films has been obtained and the edge of absorption seems to be similar to the absorption edge typical for amorphous semiconductors, what allowed to obtain the Urbach energy and E_T parameter. The energy gaps of the films, following the Tauc relation, are found to change from 2.05 to 2.4 eV, depending on the length of conjugated part of the polymer chain. Absorption bands above the absorption edge, observed for different polyazomethine films are connected with their chemical structure and possible electronic transitions.     

Observation of HfO2 thin films by deep UV spectroscopic ellipsometry

Deep UV spectroscopic ellipsometry (SE) is used for structure change observations in thin hafnia (HfO₂) layers deposited by p-MOCVD on silicon substrate. The absorption edge E_g and most of the critical point transitions in HfO₂ are above 6 eV, which makes the extension to Deep UV SE (5–9 eV) very suitable. The phase mixture changes as function of thickness and deposition process temperature, deduced from SE correspond well to XRD and Angle Resolved (AR)-XPS spectroscopy observations. From the absorption spectra at 4.5 eV, defects such as oxygen vacancies are detected, whereas from XPS spectra the estimation of the O/Hf ratio follows the same trend. Deep UV SE reveals differences in the dielectric function with orthorhombic/monoclinic phase mixtures essentially with peaks at 7.5 and 8.5 eV. Quantum confinement originated from the grain size of the films and the excitonic origin of the 6 eV feature are discussed.     

Optical properties of microcrystalline 3C–SiC:H films measured by resonant photothermal bending spectroscopy

Optical absorption coefficient spectra of hydrogenated microcrystalline cubic silicon carbide (μc-3C–SiC:H) films prepared by Hot-Wire CVD method have been estimated for the first time by resonant photothermal bending spectroscopy (resonant-PBS). The optical bandgap energy and its temperature coefficient of μc-3C–SiC:H film is found to be about 2.2 eV and 2.3 × 10⁻⁴ eV K⁻¹, respectively. The absorption coefficient spectra of localized states, which are related to grain boundaries, do not change by exposure of air and thermal annealing. The localized state of μc-3C–SiC:H has different properties for impurity incorporation compared with that of hydrogenated microcrystalline silicon (μc-Si:H) film.     

VUV and IR absorption spectra induced in H2-loaded and UV hyper-sensitized standard germanosilicate preform plates through exposure to ArF laser light

H₂-loaded Ge-doped preform plates have been UV hyper-sensitized and subsequently post-exposed by means of UV ArF laser pulses at 193 nm. Both Fourier Transform Infrared (FTIR) and Vacuum Ultraviolet (VUV) absorption spectroscopy has been carried out at each step of the sensitization process with a view to get a better understanding of the UV hyper-sensitization process at 193 nm. Exposing the H₂-loaded samples firstly triggers a partial bleaching of the 5 eV absorption band, followed by a new growth of the band for longer exposure time. As it has already been reported for exposure at 248 nm, the exposure at 193 nm firstly yields an increase in the absorption ascribed to hydroxyl (≈3600 cm⁻¹), hydride species (≈2140–2185 cm⁻¹) and GeE′ defects (≈6.3 eV). The evolution of the absorption related to the GeH₂ species is not monotonous but rather the absorption reaches a peak (N_pre = 2000) and then slightly decreases for further exposure time. Post-exposing the hyper-sensitized samples (N_pre = 2000) lead to a partial bleaching of the absorption ascribed to hydride species and to increases in the intensity of the GeE′ defect-related absorption and in the absorption ascribed to hydroxyl species. The accurate determination of the species concentration from the absorption spectra proved to be rather tricky due to the problem of accurately measuring the depth of the UV beam penetration at each time of the exposure. Nevertheless, a correlation could be established between the growth of the UV-induced UV excess loss ascribed to GeE′ species and the UV-induced decrease in the intensity of the IR bands related to hydride species. These observations are discussed within the frame-work of the two step model.     

FTIR and UV–VIS optical absorption spectra of gamma-irradiated MoO3-doped lead borate glasses

Structure and optical properties of MoO₃-doped lead borate glasses which contain high PbO content (60, 70 and 80%) have been studied using Fourier transform infrared (FTIR) and ultraviolet–visible (UV–VIS) spectroscopic tools. FTIR spectra reveal absorption bands which are characteristic for various structural units of borate network, mainly BO₃ triangles and BO₄ tetrahedra, in addition to the PbO_n (where n = 3 and/or 4) structural units. UV–VIS optical absorption spectra reveal broad intense charge transfer UV bands due to Pb^2 + ions in the range 320–385 nm. Within this range, molybdenum ions, preferably Mo^3 + and Mo^5 +, can interfere at about 360–385 nm. Additionally, molybdenum ions give a weak visible band at about 850–860 nm. The optical absorption spectra of the studied glasses show marked resistance to successive gamma irradiation up to 5 Mrad. This shielding behavior can be related to the present high content of the high atomic mass Pb^2 + ions. Changes in the atomic structure before and after gamma irradiation are described and explained.     

Nonlinear optical properties of PbS quantum dots in boro-silicate glass

PbS quantum dots synthesis in boro-silicate glass is presented. Absorption bleaching of PbS quantum dots of ≈4 and ≈7 nm in diameter dispersed in this glass has been studied. Bleaching relaxation time of 20–30 ps, absorption saturation fluence of ≈5 mJ/cm² and ground-state absorption cross-section of 2 ÷ 6 × 10⁻¹⁷ cm² at the wavelengths corresponding to the first excitonic absorption band maxima are measured.     

A study of absorption coefficient spectra in a-Si:H films near the transition from amorphous to crystalline phase measured by resonant photothermal bending spectroscopy

Optical absorption spectra of a widegap hydrogenated amorphous silicon film have been estimated by resonant photothermal bending spectroscopy. It is found that excess absorption exists over the photon energy region of 1.2–1.6 eV. This excess absorption decreases by light illumination and does not recover through thermal annealing. The decrease in the excess absorption may be due to oxidization of the film by light illumination.     

Optical absorption in amorphous InN thin films

Amorphous indium nitride (a-InN) thin films were deposited onto different substrates at temperatures <325 K using RF magnetron sputtering at a rate 0.3–0.4 Å/s. X-ray diffraction patterns reveal that the films grown on the substrates are amorphous. The optical absorption edge, ‘bandgap’ energy, E_g, of a-InN has been determined by spectroscopic ellipsometry over the energy range 0.88–4.1 eV. The absorption coefficient was obtained by the analysis of the measured ellipsometric spectra with the Tauc–Lorentz model. The E_g was determined using the modified Tauc and Cody extrapolations. The corresponding Tauc and Cody optical bandgaps were found to be 1.75 and 1.72 eV, respectively. These values are in excellent agreement with the values of the bandgap energy obtained as fitting parameters in the Tauc–Lorentz model: 1.72 ± 0.006 eV as well as by using spectrophotometry (1.74 eV) and photoluminescence (1.6 eV). The spectral dependence of the polarized absorptivities was also investigated. We found that there was a higher absorptivity for wavelengths <725 nm. This wavelength, ∼725 nm, therefore indicates that the absorption edge for a-InN is about 1.70 eV. Thus, the average value of the measured optical absorption of a-InN film is approximately 1.68 ± 0.071 eV.     

Absorption and luminescence in amorphous SixGe1-xO2 films fabricated by SPCVD

Optical absorption and photoluminescence of Ge-doped silica films fabricated by the surface-plasma chemical vapor deposition (SPCVD) are studied in the 2–8 eV spectral band. The deposited on silica substrate films of about 10 μm in thickness are composed as x·GeO₂-(1-x)·SiO₂ with x ranging from 0.02 to 1. It is found that all as‐deposited films do not luminesce under the excitation by a KrF (5 eV) excimer laser, thus indicating lack of oxygen deficient centers (ODCs) in them. After subsequent fusion of silicon containing (x < 1) films by a scanning focused CO₂ laser beam absorption band centered at 5 eV as well as two luminescence bands centered at blue (3.1 eV) and UV (4.3 eV) wavelengths arise, highlighting the formation of the ODCs. The excitation of unfused SPCVD films by an ArF (6.4 eV) excimer laser yields a luminescence spectrum with two bands typical for the ODCs, but with a faster decay kinetics. Intensities of these bands grow up with samples cooling down to a temperature of 80–60 K. Unfused films excited by the ArF laser also demonstrate luminescence due to recombination of a trapped charge resulted from the excitation of localized electron states of the glass network. In the unfused GeO₂ film luminescence related to a self-trapped exciton (STE) typical for GeO₂ crystals with α-quartz structure is observed. The observed STE luminescence can be indicative of the crystalline fraction availability in the film. Whereas GeO₂ crystals are known as not containing twofold coordinated germanium, a polycrystalline inclusion in the SPCVD GeO₂ film serves as a factor explaining the absence of any spectroscopic manifestation of this type of defects in it even after fusion. On the other hand, lack of STE luminescence in other unfused films with x < 1 testifies truly amorphous state of the matter in them.     

Effects induced by 4.7 eV UV laser irradiation on pure silica core multimode optical fibers investigated by in situ optical absorption measurements

We investigated by in situ optical absorption measurements the effects induced by 4.7 eV UV laser irradiation on pure silica core optical fibers. Laser irradiation with 100 MW cm^? 2 laser intensity generates in the fiber E′ centers which partially decay after irradiation due to their reaction with diffusing H₂. An absorption band peaked at 5.3 eV is observed to grow in the post-irradiation stage with a kinetics anti-correlated to the decay of the 5.8 eV band of the E′ centers. The defect absorbing at 5.3 eV is proposed to be formed by trapping on pre-existing precursors of hydrogen atoms made available by breaking of H₂ on E′. We also show by repeated irradiation experiments that the 5.3 eV-absorbing center is photochemically destroyed by 4.7 eV laser light, and we estimate the cross section of this process. Possible structural models for this defect are discussed.     

Investigation of the conductivity of the lithium borosilicate glass system

The lithium borosilicate system (Li₂O)_0.4(B₂O)_(0.6x)(Si₂O₄)_0.6(1−x) with x = 0, 0.2, 0.3, 0.4, 0.6, and 0.8 was investigated using impedance spectroscopy. Impedance spectra were taken in the frequency range from 50 Hz to 1 MHz and in the temperature range from 100 to 280 °C. The ac- and dc-conductivity, relaxation frequency and activation energy of the dc-conductivity were extracted from the impedance spectra. The dc-conductivity of the investigated glass samples increases almost linearly from silica rich (x = 0) to the boron rich (x = 0.8) samples. Activation energy (E_a) was found to be 0.65 eV for high conducting sample and 0.8 eV for low conducting sample, respectively. The mixed glass-former effect was not observed on the samples studied. The effect of temperature scaling of ac-conductivity was observed, which indicates, that ionic conductivity relaxation mechanism is temperature independent for samples with x = 0, 0.2, 0.3. However, some deviations from scaling were found for the samples with higher x (x = 0.4, 0.6, 0.8).     

Luminescence and absorption spectroscopy of Sn-related impurity centers in silica

We report an experimental study on the absorption and luminescence spectra of oxygen deficient point defects in Sn-doped silica. The absorption band at 4.9 eV (B_2β band) and the two related photoluminescence bands at ∼4.2 eV (singlet–singlet emission, S₁ → S₀) and at ∼3.2 eV (triplet–singlet emission, T₁ → S₀), linked by a thermally activated T₁ → S₁ inter-system crossing process (ISC), are studied as a function of temperature from 300 to 20 K. This approach allows us to investigate the dynamics properties of the matrix in the surroundings of the point defects and the effects of local disorder on the two relaxation processes from S₁: the radiative channel to S₀ and the ISC process to T₁. We observe that the S₁ → S₀ decay kinetics at higher temperatures do not follow a single-exponential law and the ISC rate shows a temperature dependence that cannot be rationalized by a single activation process, suggesting the presence of a complex landscape of configurational energies. The comparison with analogous data for Ge-doped silica reveals that the local dynamics of the matrix, the defect–matrix electron–phonon coupling, and the ISC rate dispersion are not substantially modified by the isoelectronic and isostructural substitution Sn–Ge. On the contrary, the Sn-related ISC process is ∼5 times more efficient than the Ge-related one. Since we observed that the coupling with local phonons increases the ISC efficiency by four order of magnitudes in the investigated temperature range, the reported data strongly suggest that, even if the presence of the spin–orbit coupling is needed for ISC processes, it has not play a primary role in the ISC processes in silica, where it acts as a homogenous and temperature-independent scale factor.     

Blue,green and red upconversions in Ho2O3-doped fluorophosphate glasses

Near infrared (NIR) to visible upconversions of a fluorophosphate glass of composition (mol%) 7Ba(PO₃)₂–32AlF₃–30CaF₂–18SrF₂–13MgF₂ doped with various concentrations (0.1, 0.3 and 1.0 mol%) of Ho₂O₃ have been investigated by exciting at 892 nm at room temperature. Three upconverted bands originated from the ⁵F₃ → ⁵I₈, (⁵S₂, ⁵F₄) → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions have been found to center at 491 nm (blue), 543 nm (green) and 658 nm (red), respectively. These bands have been justified from the evaluation of the absorption, normal (down conversion) fluorescence and excitation spectra. The upconversion processes have been interpreted by the excited state absorption (ESA), energy transfer (ET) and cross relaxation (CR) mechanisms involving population of the metastable (storage) energy levels by multiphonon deexcitation effect. It is evident from the infrared reflection spectral (IRRS) analysis that the upconversion phenomena are expedited by the low multiphonon relaxation rate in fluorophophate glasses owing to their high intense low phonon energy of ∼600 cm⁻¹ which is very close to that of fluoride glasses (500–600 cm⁻¹).     

Evolution of pressure-amorphized zirconium tungstate upon annealing

Zirconium tungstate Zr(WO₄)₂ exhibits irreversible amorphization at high pressure. Upon heating, the pressure-amorphized phase transforms into different phase/phases depending on the pressure applied during heating. Transformation of pressure-amorphized samples to metastable cubic phase during isochronal annealing at ambient pressure is investigated using X-ray diffraction and Raman spectroscopy. Though the X-ray diffraction and Raman spectra show only monotonic changes up to 850 K in the amorphous state, the sample exhibits dramatic changes in the color from gray to black to white. The relaxation of the amorphous phase during annealing suggests gradual irreversible volume increase by about 6%, whereas the tungstate tetrahedra are found to shrink. Crystallization to cubic phase at 900 K is accompanied by a large increase in the sample volume. The specific volumes of the amorphous phases obtained from cycling the samples to different pressures suggest the possibility of polyamorphism in this system.     

Absorption,emission and absorption saturation of Cr4+ ions in calcium aluminate glass

Absorption, luminescence and absorption saturation of Cr ions in a calcium aluminate glass are studied. In the absorption spectrum, the absorption bands of Cr³⁺, Cr⁴⁺ and Cr⁶⁺ ions are revealed. The emission spectrum presents luminescence of Cr³⁺ ions centered at 0.82 μm and that of Cr⁴⁺ ions at 1.3 μm. The luminescence signal demonstrates short decay times of 120 ± 10 ns and 300 ± 20 ns for Cr³⁺ and Cr⁴⁺ ions, respectively. Absorption saturation measurements allowed an estimate of the ground-state absorption cross-sections for Cr⁴⁺ ions at 1.06 μm of 0.7 × 10⁻¹⁸ cm² and at 0.69 μm of 1.5 × 10⁻¹⁸ cm².     

Solid phase epitaxy of Na-ion irradiated α-quartz: Cathodoluminescence,kinetics and surface morphology

Even at low fluences doping of quartz by ion implantation results in amorphization. Here we report on the measurement of cathodoluminescence and surface structures during solid phase epitaxy in Na-implanted α-quartz annealed in ¹⁸O₂ atmosphere. Complete epitaxy was achieved under appropriate conditions of ion fluence, annealing temperature and time. The crystalline structure of the samples was studied by Rutherford backscattering channelling spectroscopy and the ¹⁸O–¹⁶O exchange between the matrix and ¹⁸O₂ annealing gas by elastic recoil detection analysis. In the cathodoluminescence spectra taken at room temperature, five bands were identified and assigned to various defect centres. Two of the bands in the violet region at 3.25 and 3.65 eV strongly vary in intensity at 843–1173 K annealing temperature and appear to be intimately correlated with the presence of Na-ions in the implanted region of the matrix. Finally, atomic force microscopy enabled, for the first time, the observation of the correlation of surface structures and epitaxy.